Six Secrets to Soil Test Success (Part 2)

09 June 2015

In the first part of this article, I discussed the mechanics of three ratios that play a major role in soil health and crop production. In the second part of this article we will look at the remaining three key ratios, which include: the phosphorus to zinc ratio, the potassium to sodium ratio and the iron to manganese ratio.

Getting the Energy Minerals Right

Phosphorus (P) is called "the energy mineral" because it is the building block for ATP (adenosine tri-phosphate), which drives every enzymatic reaction. ATP is, in effect, the battery of life, because enzymes drive all biological reactions and life stops without ATP. Phosphorus is also the major mineral required for plant immunity and the production of glucose from photosynthesis is largely based on phosphate-based enzymes.

Zinc (Zn) is called "the energy micronutrient" because this mineral is required, in the right balance with P, to ensure that phosphate energises as it should. Zinc is also linked to moisture uptake and the performance of nitrogen-fixing organisms in the root zone. However, most importantly, this critically important trace mineral is required for plant and soil organisms to produce auxins.

Auxins are a group of hormones produced by the plant and beneficial microorganisms (to support their host, the plant), which provide a number of essential benefits. The most important of these relates to leaf size. The leaf is the solar panel that determines photosynthetic performance and zinc governs leaf size. A zinc deficiency spells a substandard leaf, less glucose production and an inevitable yield reduction. This is why zinc is often considered to offer the best cost to benefit ratio of any trace mineral. A deficiency will always be costly and yet it is relatively inexpensive to address.

The key is to supply both phosphorus and zinc in the ratio that ensures maximum performance of both minerals. In this instance, that ratio is actually more important than the numbers game. Ten parts phosphorus to one part zinc is the proven productive ratio between these two minerals. If you had 30 ppm of P on your soil test, for example, and 3 ppm of Zn, both minerals are technically deficient. However, the ratio between them is correct (10:1). Maintaining that ratio is the key with soil correctives.

It would be very counterproductive if you were to lift zinc levels to the minimum required level of 5 ppm (because it is less costly to address Zn than P) but ignore the phosphate correction. Your perfect ratio is now compromised (it would be reduced to 6:1), which is worse than no action at all. If you have a limited budget, then limit your correction accordingly. However, always ensure that you maintain the all-important 10:1 phosphorus to zinc ratio. When we are talking about the mineral ratios relative to phosphorus, I should clarify the P extraction involved. I am referring specifically to Mehlich 3 extraction, as practised by EAL Lismore and US lab, Brookside Laboratories.

Balancing The Lookalike Cations

Potassium and sodium are two of the major cations that are stored in greatest abundance on the clay colloid in our soils. "Base saturation" on your soil test refers to the relative percentages of the base cations, including calcium, magnesium, potassium, sodium and hydrogen, that are attached to the clay component of your soil. Ideally, we should aim for 3 – 5% saturation of potassium (3% for pasture and broadacre crops and 5% for more intensive horticulture).

However, sodium is required at less than 25% of this rate. Sodium should never exceed 1.5% on the base saturation portion of your soil test, but more importantly, you should never have a higher percentage of sodium than potassium. Should this occur, the plant may have problems differentiating between these lookalike minerals. The plant seems to assume that potassium will naturally be present in higher amounts, so it simply extracts the mineral that is present at the higher percentage at the time. If that is sodium, there is a price to pay. Sodium expands in the heat, bursts cell walls, and you have burnt edges on your leaves. Unfortunately, this is not just a cosmetic issue. The plant no longer has the required amount of potassium to transfer sugars, open stomates, sweeten fruit, size up fruit and grains, strengthen stems etc, etc. You will suffer yield and quality limitations as a result of this imbalance. The key is to always maintain a higher percentage of potassium than sodium in terms of base saturation. The ideal K:Na ratio may be around 4:1, but the critical thing is to ensure that sodium levels are never higher than potassium.

Ensuring Adequate Supply of the Immune Enhancers

Iron and manganese are essential trace minerals for plant resilience. The plant uses these minerals for many of the compounds it creates to defend itself against marauding microbes and insects. The iron to manganese ratio is the last of our six soil secrets. The goal here is to always ensure that your soil contains more parts per million (ppm) of iron than manganese. However, this ratio should never exceed 2 parts of iron to one part of manganese, or you may induce a manganese deficiency. Iron and manganese are antagonistic to each other when oversupplied, so a manganese excess can also induce an iron deficiency. The key is to achieve the desirable balance, where there is more iron than manganese, but never more than 2:1. If we were to be less specific, the most important thing here is to just make sure that iron is higher than manganese at all times.

Note: It is not always certain that a poor balance of these minerals in the soil will negatively impact the uptake of either, because various factors including soil type, environmental conditions and organic matter levels can also be involved. This caution is actually applicable to all of the six ratios and that is why we always suggest the use of leaf analyses to confirm a potential problem.

In Conclusion

If you can work toward improving these ratios and monitor your success with regular soil testing, the benefits will flow. Your soil will breathe better, biology will work better and resilience, nutrient uptake, production and profitability will increase. If you can not tick off improvements in at least three of these ratios each year, then you might need to seek a new consultant or improve your own knowledge of mineral balance requirements. I wish you all happy, productive soil improvements in this, "The International Year of Soils".

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